The present invention relates in general to the detection of the peak values of an AC signal and in specific to the detection and measurement of the response envelope of an AC signal as a function of time both rapidly and with high precision.
Prior methods of obtaining the envelope of an electrical signal rely upon: 1) the use of diode detector circuits incorporating an RC time constant; 2) digitizing the wave form of the signal and performing a mathematical fit to determine each peak value or 3) making measurement directly from an oscilloscope trace.
The first method which employs a diode detector circuit is limited in its accuracy because of the response characteristics of the RC circuit. Since the RC interaction includes a relatively substantial time delay, necessary for the charging and discharging of the capacitor the accuracy of the measurement is limited.
The second method, digitizing of the wave form, require substantial amounts of numerical manipulation of the data and requires a multitude of mathematical operations. This method also requires performing a mathematical xe2x80x9cfitxe2x80x9d which introduces error into the envelope measurement.
The third method is simply slow and inaccurate because it requires an operator to visually plot the decay response of the signal. All of the above techniques have limitations that are overcome by the new method.
The major improvement introduced by the new method is a large increase in the precision of measurements which results in correspondingly large increases in sensitivity of instruments employing the new technique.
It is an object of the present invention to provide a method for the efficient and accurate measurement of the envelope of an AC signal.
It is also an object of the present invention to provide a system Which facilitates the accurate measurement of the peak amplitudes of an AC signal as a function of time and thus determine the envelope of the signal.
It is a further object to provide a means by which the decay response of an AC signal can be efficiently and accurately measured and recorded.
It is also an object to provide an accurate method for monitoring the rate of energy dissipation of a vibrating solid.
It is a further object to provide a device which provides the highest possible resolution in mechanical loss data in order to provide the highest sensitivity and selectivity from resonator based sensors.
These and other objects are accomplished with a digital envelope detector, consisting of both hardware and software, that provides accurate measurements of changes of peak values of an AC signal. These peak values constitute the envelope of a signal. Such accurate envelope measurements are required, e.g., to optimize the accuracy and selectivity of chemical sensors. The envelope values required for these sensors can not be obtained with common instruments (e.g. voltmeters) since these meters require that successive peaks be the same amplitude. Therefore, they can not measure the envelope of a gradually increasing or decreasing AC signal from the chemical sensors.
Prior to the instant invention, the only possible alternative was the use of high speed, high resolution analog-to-digital conversion (ADC) followed by extensive statistical analysis. The analog-to-digital conversion method is much more expensive, slower, and excessively complicated compared to the instant invention. Applicant""s invention works as follows: A signal of is interest compared to each of a set of accurately calibrated reference (or threshold) voltages provided by a digital to analog converter. A digital logic circuit and software respond each time the signal fails to exceed the current reference voltage. If and when the monitored signal fails to exceed the reference voltage, relevant data (e.g. time or cycle count) are digitally recorded and a new reference voltage is installed. The process is repeated until the desired range of change of the signal is measured. The result is a set of amplitudes as a function of time and/or cycle that fully and accurately describe the desired portion of a signal envelope. The method for obtaining the highest resolution values for the envelope of an electrical signal are described by explaining its operation when it is applied to obtain the envelope of a monotonically decreasing signal, however this technique is not limited to this particular application.